This invention relates to photographic apparatus and in particular to a compact, rigid camera that employs a prism in the optical image path. The invention is particularly suited for use with cameras that use self-developing film and accordingly is described with particular reference to such cameras. Features of the invention can, however, be used in other cameras.
Conventional snapshot cameras derive compactness principally from the use of film having a relatively small frame format and, in some instances, from a collapsing bellows arrangement. For 35-millimeter cameras, the film frame is typically 24 millimeters by 36 millimeters. Since the focal length of the objective lens of the camera is usually at least as long as the diagonal of the film frame to provide a useful angular field, the lens-to-film plane distance would then be of the order of at least 43 millimeters (approximately 1 and 11/16 inches). (Of course, wide angle lenses for 35 millimeter cameras can have a significantly shorter focal length.) In conventional cameras where the light from the lens follows a straight path in air to the film, the focal length is therefore a controlling limitation on the front to back dimension of the camera. A feature of this construction is that there is room for only one or two optically active surfaces between the lens and the film which can reflect or scatter light. This and the relatively small film format reduce the likelihood that stray light from outside the angular field of the lens, or other unwanted light, will reach the film.
The difficulties in achieving a camera having a compact size are greatly increased when the camera uses self-developing film, such as that marketed by the Polaroid Corporation. While in conventional photography, a relatively small negative can produce a relatively large print or viewable image by enlargement or projection, the exposed photographic area on self-developing film must, in most instances, be the same as that of the actual developed print. As a result, instant cameras, i.e. designed for use with self-developing film generally have a significantly longer focal length than that in compact cameras using conventional film. Also, instant cameras generally require a fairly high speed lens to provide sufficient light to expose the film at exposure times useful in hand held cameras for general purposes. These requirements for the exposure of self-developing film therefore present significant constraints on the design of a compact instant camera.
Early solutions to the problem of achieving compactness in an instant camera involved mechanical folding arrangements to move the objective lens in a direction perpendicular to the film plane. A folding bellows enclosed the camera space between the objective lens and the film plane. More recently, cameras developed and marketed by the Polaroid Corporation utilize a reflective element in the exposure chamber to fold the optical path between the lens and the film plane. A collapsible version of such a camera which achieves a high degree of compactness is described in U.S. Pat. No. 3,753,392. Another instant camera of this type which employs a reflective element is described in U.S. Pat. No. 3,938,167 and U.S. Pat. No. 3,940,774. This camera is noncollapsible, or rigid, and therefore has a lower cost of manufacture than the collapsible camera, but suffers from a comparatively bulky configuration for carrying purposes.
One problem with some of these cameras employing a reflective element is that the film upper surface, which typically is a thin sheet of transparent plastic material, reflects a portion of the light incident on the film. The reflective element redirects this film-reflected light back onto a different part of the film, causing overexposure at that part and loss of sharpness in the resultant photograph. While this problem is not severe in many situations, it can produce noticeable "flare" spots when the camera is used outdoors with a bright sky and is particularly acute when the subject is relatively dark.
U.S. Pat. No. 3,818,498 of Zehnpfennig discloses a compact camera designed for self-developing film and which employs a pair of spaced apart, mutually inclined reflective elements to generate a multiply-folded optical path. One of the elements is fully reflective, while the other is partially reflective. The partially reflective element overlies a selector element which overlies the film and is formed of mechanical light collimators held in a transparent medium. While this arrangement may achieve a highly folded optical path, it nevertheless suffers from several disadvantages. First, at least a portion of the light reaching the film plane undergoes multiple reflections from the partially reflective element. At each reflection the incident light looses a significant portion of its intensity. As a result, the intensity of the light reaching the film plane is generally low and of varying values depending on the number of reflections the light has undergone. Another problem is that the mechanical collimators of the selector element cast a shadow on the film plane or produce granularity, or defocus, or some other form of image degradation.
While the light folding properties of prisms are well known, the principal uses of prisms have been in nonphotographic optical instruments such as binoculars, telescopes, periscopes, rangefinders and spectrometers. Many applications rely on the ability of a prism to redirect by total internal reflection light incident on a surface adjacent a medium of a lower index of refraction at an angle greater than a critical value. Common prisms which utilize this property are the Porro prisms (of first or second kinds) commonly employed in binoculars. Other common prism configurations such as Dove, Lehman and Amici prisms use total internal reflection for image inversion, field rotation or scanning. In these applications, while the incident light beam is reflected one or more times, and hence is to some extent "folded," the main purposes of the prism element are not to fold the optical path to achieve compactness, but rather to redirect, laterally displace, invert, split, combine or rotate the beam or beams.
One characteristic of a Schmidt prism, which is another prism that provides multiple internal reflections, is that a portion of one prism face can provide total internal reflection while another portion is transmissive. Schmidt prisms, single and in matched pairs, also have found applications in optical instruments. A discussion of some applications can be found in "Optical Systems for Telescopes and Binoculars" at pp. 435-471 of Summary Technical Report of Division 16, NDRC, Vol. I, Optical Instruments (1946). U.S. Pat. No. 3,471,685 to Kato et al discloses a matched pair of Schmidt prisms operating as a field rotator in a microscope. In Kato, as is common with optical instruments such as telescopes and periscopes, photograph apparatus can be attached to the eyepiece to record the output image of the instrument.
Heretofore, prisms have been used in cameras principally as image directing elements in viewfinders. For example, many 35-millimeter single lens reflex cameras employ a roof pentaprism to direct light from a deviating mirror to the viewfinder eyepiece. U.S. Pat. No. 3,819,255 to Matui discloses a more complex viewfinder structure employing an opposed pair of Schmidt prisms that are mutually rotatable about a fixed pivot with an air gap separating the opposed faces. A portion of each opposed face internally reflects the incident light beam and another portion transmits or receives the light beam. It is noteworthy, however, that the light transmission through these prisms is over a relatively small portion of the opposed prism faces. Further, the light transmission to a viewfinder does not require the optical quality or transmission efficiency necessary for light transmission to photographic film. Also, unwanted or stray light, and the loss of light intensity, are not as critical in viewfinder optics as in the image-path optics of a camera.
U.S. Pat. No. 3,784,645 to Grey and U.S. Pat. No. 3,911,692 to Grey et al disclose prism elements located within the exposure chamber of a camera and forming part of the optical path between the objective lens and the film plane. More specifically, these patents teach that prism elements be provided in a stereoscopic camera to lateraly displace two light beams, each originating at a separate objective lens, so that they are recomposed in a side by side relationship on two halves of a single film frame. This displacement function is roughly analogous to that of roof prism pairs in binoculars. These patents also deal with numerous optical design problems generated by the prism elements, including such prism characteristics as distortion, astigmatism, chromatic aberration, spherical aberration, light absorption, the weight of the prism elements, and the elimination and/or control of stray light. Another design consideration is that the use of prisms in cameras increases the back focal distance for a given focal length. The ability of a prism to fold the optical path particularly with a high index prism, however, can more than offset this increase. It should also be noted, however, that the " foldability" does not necessarily result in compactness.
In particular, these disclosures teach the desirability of spacing the exit face of each prism at a practical distance from the film plane to avoid abrasion and to avoid having dust or other irregularities present on the face of the prism cast a shadow on the film. The large spacing also has the advantage of allowing the use of smaller prisms, which in turn reduces the absorption of the light within the prism and reduces the weight which the prism elements add to the overall optical system. It is also clear from these disclosures that a material such as glass with a relatively high index of refraction, typically 1.6 or 1.7, is preferable to a plastic material -- typically with an index of refraction only near 1.5 -- both in terms of optical efficiency and of "foldability."
With this state of the art, it is a principal object of this invention to provide a camera for use with self-developing film and which is both rigid and highly compact.
Another object of this invention is to provide a camera that employs, in the image path, a prism having relatively few optically active surfaces and few critical tolerances.
A further object of the invention is to provide a rigid highly compact prism camera that effectively controls unwanted light.
Another object is to provide a camera with the foregoing advantages and which utilizes a prism of plastic material.
It is also an object of the invention to provide a compact instant camera that greatly reduces "flare" problems due to light reflected from the film surface while having an improved level of image brightness and uniform light distribution.
A further object of the invention is to provide a highly compact instant camera that allows flexibility in the alignment of the optical axis of the objective lens with respect to the film plane.
Still another object of the invention is to provide a rigid, highly compact instant camera having a stream-lined, aesthetically pleasing configuration that is convenient to use, carry and store.